This invention relates to the field of color printers or writers and is specifically concerned with writing digital color images onto motion picture film for theater projection.
For quite some time writers have existed that can take electronic/digital image data and use it to expose color motion picture film. The most mature of the technologies, still in use today, uses the color cathode ray tube (CRT). Celco is one of several manufacturers in this field. Another is Management Graphics which makes the Solitaire CRT film writer. The electron beam generated by the CRT strikes the red, green, and blue (RGB) phosphors on the surface of the tube. The phosphors then emit light which is imaged onto the film. The image is written in a raster scan.
Laser writers for film, capable of higher resolution and generally higher speeds than their CRT counterparts, have been in use for a decade. The Kodak Cineon System uses a laser writer with red, green, and blue gas laser sources. The lasers are raster scanned by a rotating polygon mirror onto a moving drum. A more recent introduction, the Arrilaser by Arri, uses a flat platen instead of a drum and a single faceted scanner mirror (monogon). The red, green, and blue lasers use solid state or diode laser technology, but the basic approach is the same. Three primary color sources, red, green, and blue, are used to expose the three emulsion layers (red sensitive, green sensitive, and blue sensitive) to produce three color dye layers in the developed film. Typically, in a color negative media, the dyes are the subtractive color primaries: cyan, magenta, and yellow.
For example, U.S. Pat. No. 6,018,408 discloses a RGB raster scan laser projector using polygon/galvo scanner. A white light laser is separated into RGB components for modulation and projection. Similar devices are designed to demagnify the image for film exposure.
As another example, U.S. Pat. No. 5,537,258 discloses a laser projection system with red, green, and blue dye lasers providing the primary colors for forming an image using a single shared spatial light modulator. In this case, instead of a raster scan, the entire image area is exposed simultaneously, by using an area modulation device.
FIG. 1 shows a familiar color gamut representation using CIE 1976 L*u*v* conventions, with the perceived eye-brain color gamut in uxe2x80x2-vxe2x80x2 coordinate space represented as a visible gamut 100. Pure, saturated spectral colors are mapped to the xe2x80x9chorseshoexe2x80x9d shaped periphery of the visible gamut 100 curve. The interior of the xe2x80x9chorseshoexe2x80x9d contains all mappings of mixtures of colors, such as spectral red with added blue, which becomes magenta, for example. The interior of the horseshoe can also contain mixtures of pure colors with white, such as spectral red with added white, which becomes pink, for example. The overall color area defined by the xe2x80x9chorseshoexe2x80x9d curve of visible gamut 100 is the full range of color that the human visual system can perceive. It is desirable to represent as much as possible of this area in a color display, to come as close as possible to representing the original scene as we would perceive it if we were actually viewing it.
The gamut available using conventional color motion picture film is shown by a conventional motion picture film gamut triangle 102 in FIG. 1. The approximate wavelengths of vertices of the triangle are shown as red (620 nm), green (540 nm), and blue (455 nm). Any color within the bounds of this triangle can be displayed. Colors lying outside the triangle but within the xe2x80x9chorseshoexe2x80x9d curve can be perceived by the human eye but cannot be represented with conventional color film. Such colors are out-of-gamut colors, such as turquoise (blue-green), for example.
FIG. 2 shows the same curve 100 with the human eye response, but this time a polygon 106, representing the gamut achievable using a four color laser display is shown. The vertices of gamut polygon 106 are the laser wavelengths: red 116 at 649 nm, green 114 at 514 nm, blue-green 112 at 488 nm, and blue 108 at 442 nm. Lasers, by their very nature, are monochromatic, providing fully saturated colors, unlike most light sources. Saturated colors lie on the periphery of the xe2x80x9chorseshoexe2x80x9d curve. The resulting four laser gamut covers virtually the whole range of visual color space. Clearly, the introduction of a fourth color into this display provides a considerable gamut increase over that of conventional motion picture film as shown in FIG. 1.
With respect to digital projection apparatus, there have been some attempts to expand from the conventional three-color model in order to represent color in a more accurate, more pleasing manner. Notably, few of these attempts are directed to expanding the color gamut. For example, U.S. Pat. No. 6,256,073 (Pettit) discloses a projection apparatus using a filter wheel arrangement that provides four colors in order to maintain brightness and white point purity. However, the fourth color added in this configuration is not spectrally pure, but is white in order to add brightness to the display and to minimize any objectionable color tint. It must be noted that white is an xe2x80x9cintra-gamutxe2x80x9d color addition; in terms of color theory, adding white actually reduces the color gamut by desaturating the color. Similarly, U.S. Pat. No. 6,220,710 (Raj et al.) discloses the addition of a white light channel to standard R, G, B light channels in a projection apparatus. As was just noted, the addition of white light may provide added luminosity, but constricts the color gamut.
U.S. Pat. No. 6,191,826 (Murakami et al.) discloses a projector apparatus that uses four colors derived from a single white light source, where the addition of a fourth color, orange, compensates for unwanted effects of spectral distribution that affect the primary green color path. Again, the approach disclosed in the Murakami patent does not expand color gamut and may actually reduce the gamut.
Patent Application WO 01/95544 A2 (Ben-David et al.) discloses a display device and method for color gamut expansion using four or more primary colors. However, the approach disclosed in WO 01/95544 is directed to apparatus for projection of digital images, but does not provide a suitable solution for imaging onto a photosensitive medium. It must be emphasized that there are significant differences between display and printing of digital color images. For example, image brightness, which must be optimized in a display system, is not a concern in printing apparatus design. Resolution, on the other hand, while not as important for images displayed on-screen, is very important for images printed on film or paper. Timing requirements are not as demanding for color printing, since successive exposures can be used for successive layers of a photosensitive medium. Notably, the apparatus disclosed in WO 01/95544 forms an image by projecting four colors, but uses three-color RGB data as input for computing a four-color value. It can be appreciated that there would be advantages in obtaining and processing four-color data throughout the imaging process, rather than using interpolation algorithms to compute a fourth color coordinate from three-color data.
It would be advantageous to have a color film equivalent to the extended gamut of a four laser display as represented in FIG. 2. Digital cinema, now in its infancy, can take immediate advantage of this increased gamut to enhance the theatrical experience of the movie audience. Although digital projection may gradually replace many of the 35 mm film projectors in existence today, it would be economically advantageous for filmmakers to have the capability to have their movies shown on film projectors as well as on digital cinema projectors, and NTSC and HDTV television. However, merely exposing a conventional color film to these four laser sources would not change the gamut available beyond that of FIG. 1.
An object of the present invention is to provide a color printer for photosensitive media that provides four different color light sources to print on photosensitive media that has four separate spectral sensitivities depositing four dyes upon processing to expand the color gamut of the resultant image.
Briefly, according to one aspect of the present invention a color printer for printing to a photosensitive medium comprises a first light source for generating a first color beam and a first modulator for modulating the first color beam. A second light source for generating a second color beam and a second modulator for modulating the second color beam. A third light source for generating a third color beam and a third modulator for modulating the third color beam. A fourth light source for generating a fourth color beam and a fourth modulator for modulating the fourth color beam. An optical system combines and images the modulated beams onto the photosensitive medium.
It is an advantage of one embodiment of the invention to provide a compact unit for high speed writing, having four light sources and four modulators in a single plane.
It is an advantage of another embodiment to provide to provide a simple printer that combines four light sources and a single modulator in a single optical path to provide sequential exposure.
It is an advantage of another embodiment to provide a compact sequential writer that uses an x-cube to combine four light sources.
It is an advantage of yet another embodiment to provide a writer with high optical efficiency by utilizing four polarized lasers as light sources.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.